Jet engine components and other structural components used in high temperature applications generally require the use of alloys possessing high hot strength, excellent weldability, machinability, fabricability and resistance to failure by thermal shock or thermal fatigue. Over a period of several decades service performance requirements for jet engines and their components, in particular, have increased. For example, a comparison of aircraft engines in service today with those at the beginning of the jet age indicates that thrust-to-weight ratios have tripled, the time between overhauls has been increased over a hundredfold, and turbine inlet temperatures have increased from less than 1500.degree. F. to greater than 2000.degree. F. These factors have drastically increased the service requirements of jet engine component materials.
Jet engine components have been manufactured from cobalt-base superalloys containing about 40% to 63% Co plus Cr, less than about 3% Fe and large amounts of such elements as Mo, W, Cb and Ta and up to about 30% Ni. Other cobalt-containing superalloys have been developed around a base of typically 50% to 75% Ni, about 10% to 20% Co, 10% to 20% Cr, plus Ti and/or Al along with elements from the Mo, W, Cb and Ta group and less than about 7% Fe. As these alloys contain significant quantities of the moderately scarce elements nickel and cobalt, much development effort has centered around the series of heat-resistant, corrosion-resistant superalloys containing about 18% or more iron as a partial replacement for nickel and cobalt.
Among the specific factors of concern in the development of alloys for use in jet engine and other high-temperature environments is the increasing use of fuels high in sulfur, sodium and vanadium. As service temperatures have climbed, the use of these fuels has significantly increased the risks of failure due to hot gas corrosive attack. For example, sulfur and sulfur-containing compounds in jet engine gases contacting the surface of high molybdenum content alloys may disadvantageously result in the formation of low melting point sulfides. These sulfides tend to flux away the protective chromium oxide coating of these alloys.
In Table I are listed the compositions of two series of experimental alloys previously introduced for use in high-temperature, corrosive environments, in particular, for service in gas turbine rotors, shafts, blades, buckets, bolts, tailpipes, tail cones, afterburner parts, exhaust manifolds, combustion chambers, waste heat boilers, incinerators, industrial fans, furnace hardware and structures.
TABLE I ______________________________________ NOMINAL CONTENT OF ALLOY MAJOR ELEMENTS, WEIGHT % DESIGNATION Ni Co Cr Mo W Cb N Fe ______________________________________ S-495 20 -- 15 4 4 4 -- 50 S-588 20 -- 18.5 4 4 4 -- 47 S-497 20 20 15 4 4 4 -- 31 S-590 20 20 20 4 4 4 -- 25 S-816 20 45 20 4 4 4 -- 4 N-153 15 13 16 3 2.2 1 0.07 47 N-154 24 21 16 3 2.2 1 0.07 30 N-156 33 24 16 3 2.2 1 0.04 18 N-155 20 20 21 3 2.5 1 0.15 30 ______________________________________
The only alloys of this group believed to have achieved significant use and which remain available are S-590, S-816 and N-155.
Of these alloys, S-590 and N-155 have been indicated to be useful for moderate stress applications and for intermittent service up to about 1900.degree. F. However, both S-590 and N-155 begin to corrode rapidly in oxidizing combustion gases at temperatures over 1600.degree. to 1650.degree. F., and are very rapidly attacked above 1650.degree. F. in the presence of gases containing sulfur or sulfur products. Despite the fact that the S-590 alloy contains almost twice the total content of three elements thought to improve hot strength, molybdenum, tungsten and columbium, as compared to N-155, the hot strengths of S-590 and N-155 are very nearly equal at temperatures between 1200.degree. F. and 1800.degree. F. Additionally, the presence of the strong ferrite formers, molybdenum, tungsten and columbium, in the fairly large amounts of S-590 and N-155, tends to limit the level of chromium that may be present while retaining the very important stable austenitic matrix structure. Accordingly, if one increases the chromium content of this type of alloy in order to increase hot corrosion resistance, one must either increase nickel and/or cobalt content, increase carbon content, or decrease the total content of the strengthening elements such as molybdenum, tungsten and columbium in order to avoid the risk of ferrite formation. However, it is economically undesirable to increase nickel or cobalt content, and increasing carbon content may nullify other desirable properties of this type of alloy. It is also desirable to significantly reduce or remove molybdenum from these alloys type for even further enhancement of hot corrosion resistance, such as in the presence of sulfur gases or compounds. It remains desirable, therefore, to increase chromium content by some method superior to those methods described hereinabove, and to do so without loss of hot strength.
Of the alloys of Table I, S-816 has by far the highest combined nickel and cobalt content and would therefore be expected to be the most expensive. S-816 is therefore employed only in relatively small amounts as compared to other heat-resistant superalloys. Additionally, S-816 is less ductile and has lower resistance to thermal shock and fatigue than certain other heat-resistant superalloys.
Alloys S-495 and S-588 would be expected to be structurally unstable at temperatures typical of present day jet engine environments. These alloys, along with S-497, N-153, N-154 and N-156, all have such low chromium contents as to be limited to use at relatively low service temperatures.
In U.S. Pat. No. 5,077,006, it is disclosed that additions of small amounts of the six components, molybdenum, tungsten, columbium, titanium, zirconium and rare earth elements, increase hot strength of heat-resistant alloys. It is also noted that certain alloys disclosed in U.S. Pat. No. 5,077,006 incorporated additions of these elements in alloys containing up to about 25% Co. There is no suggestion in U.S. Pat. No. 5,077,006, however, that the inclusion of molybdenum is not desirable for improving hot strengths in alloys having more than about 15% Co. As described hereinbelow, however, I have now found that the inclusion of molybdenum is not desirable for improving hot strengths in alloys having more than about 15% Co. Furthermore, I have found that zirconium is not especially effective for improving the hot strength of cobalt-bearing, heat-resistant alloys having relatively low carbon contents.